chap. xin. Tierra del Fuego. 44 1 



bat farther westward the line becomes WNW. and ESE., 

 and even still more northerly. The cleavage-planes of 

 the clay-slate are highly inclined, generally at an angle 

 of above 50°, and often vertical; they strike almost 

 invariably in the same direction with the quartz ranges. . 

 The outline of the indented shores of the two main 

 islands, and the relative positions of the smaller islets, 

 accord with the strike both of the main axes of elevation 

 and of the cleavage of the clay-slate. 



Tierra del Fuego. — My notes on the geology of 

 this country are copious, but as they are unimportant, 

 and as fossils were found only in one district, a brief 

 sketch will be here sufficient. The east coast from the 

 Straits of Magellan (where the boulder formation is 

 largely developed) to St. Polycarp's Bay is formed of 

 horizontal tertiary strata, bounded some way towards 

 the interior by a broad mountainous band of clay-slate. 

 This great clay-slate formation extends from St. Le 

 Maire westward for 140 miles, along both sides of the 

 Beagle Channel to near its bifurcation. South of this 

 channel, it forms all Navarin Island, and the eastern 

 half of Hoste Island and of Hardy Peninsula ; north of 

 the Beagle Channel it extends in a north-west line on 

 both sides of Admiralty Sound to Brunswick Peninsula 

 in the Straits of Magellan, and I have reason to believe, 

 stretches far up the eastern side of the Cordiilera. The 

 western and broken side of Tierra del Fuego towards 

 the Pacific is formed of metamorphic schists, granite 

 and various trappean rocks : the line of separation be- 

 tween the crystalline and clay-slate formations can 

 generally be distinguished, as remarked by Captain 

 King, 1 by the parallelism in the clay-slate districts of 

 the shores and channels, ranging in a line between 

 [W. 20° to 40° K] and [E. 20° to 40° S.]. 



1 ' Geographical Journal,' vol. i. p. 155. 



